XFCE 4 is that way for most things that I've tried right now: The menu panels and desktop context menus work on button-down.

Switching Chrome tabs works on button-down. Activating the three-dot menu in Chrome also works on button-down (which is good) but then it needlessly fades from 0% to 100% opacity (which is a crime against nature).

I didn't even have to go digging in the memory hole to find this. It's right here in front of me.

Sure, many menus open on mousebutton-down events, but nothing is activated or started until the mousebutton-up event.

> Switching Chrome tabs works on button-down.

True; the same in Firefox. That is interesting. Perhaps it’s because simply selecting a tab is considered a safe and reversible operation. I did use the word “decisive” for a reason.

In fact, specifically for context menus this is a feature: In KDE ones, you can button-down to open the context menu and then release above the menu entry to activate it, making the entire process a single-click affair. In standard Windows ones it takes two full clicks, which is one of those tiny inconveniences that drives me crazy when using it. Of course, the Win way also works in KDE.

Windows does this for top-level menus for the same reason.

Or at least it did in classic Win32 UI. You can still see this in action if you open, say, Disk Management. But in Win11 Notepad (which is modern XAML), holding mouse down will open the drop-down submenus, but you won't actually be able to activate an item by releasing the mouse button while hovering over it, so it seems that someone partially copied the design without understanding its purpose.

Same in macOS, long press can substitute for right click in some things I’ve tried. So obviously you can’t use mouse down for that unless you you have a “time out” which defeats the purpose. Button presses are more complicated that what people expect, especially on embedded systems.

Don't games do this all the time?

I’m not sure games count as GUIs as pertaining to this discussion. GUIs in games would qualify, sure.